Imaging lens and imaging apparatus

ABSTRACT

An imaging lens is constituted by, in order from the object side to the image side: a front group; an aperture stop; and a rear group having a positive refractive power. The front group is constituted by, in order from the object side to the image side: at least two negative lenses, and a cemented lens formed by cementing a negative lens and a positive lens having a smaller Abbe&#39;s number with respect to the d line than the negative lens, provided in this order from the object side to the image side, together. The rear group is constituted by, in order from the object side to the image side: at least one positive lens, and a cemented lens having a positive refractive power as a whole, formed by cementing a positive lens and a negative lens, provided in this order form the object side to the image side, together.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2015-144898 filed on Jul. 22, 2015. The above application is hereby expressly incorporated by reference, in its entirety, into the present application.

BACKGROUND

The present disclosure is related to an imaging lens and an imaging apparatus. More specifically, the present disclosure is related to an imaging lens which can be favorably utilized in an endoscope, a vehicle mounted camera, a surveillance camera, etc., and to an imaging apparatus equipped with this imaging lens.

Conventionally, endoscopes of the insertion type, in which an elongate insertion portion having an imaging device built in at the distal end portion thereof is inserted through the mouth or the nose of a patient to image the interiors of body cavities, are in common use in the medical field. Known imaging lenses which are utilizable in such endoscopes are disclosed in Japanese Patent Nos. 5006476 and 5566560, for example.

In addition to these endoscopes, there are known fish eye lenses which are utilizable in vehicle mounted cameras, surveillance cameras, and the like as disclosed in Japanese Patent Publication Nos. S51 (1976)-002826 and S51 (1976)-014017.

All of Japanese Patent Nos. 5006476 and 5566560 and Japanese Patent Publication Nos. S51 (1976)-002826 and S51 (1976)-014017 disclose lens systems constituted by a front group, an aperture stop, and a rear group.

SUMMARY

However, correction of aberrations is insufficient in the lens systems disclosed Japanese Patent Nos. 5006476 and 5566560 and Japanese Patent Publication Nos. S51 (1976)-002826 and S51 (1976)-014017. Therefore, there is demand for an imaging lens, in which various aberrations are favorably corrected.

The present disclosure has been developed in view of the foregoing circumstances. The present disclosure provides an imaging lens, in which aberrations are favorably corrected, as well as an imaging apparatus equipped with this imaging lens.

The imaging lens of the present disclosure consists of, in order from the object side to the image side:

a front group;

an aperture stop; and

a rear group having a positive refractive power;

the front group consisting of, in order from the object side to the image side: at least two negative lenses, and a cemented lens formed by cementing a negative lens and a positive lens having a smaller Abbe's number with respect to the d line (wavelength: 587.6 nm) than the negative lens, provided in this order from the object side to the image side, together; and

the rear group consisting of, in order from the object side to the image side: at least one positive lens, and a cemented lens having a positive refractive power as a whole, formed by cementing a positive lens and a negative lens, provided in this order form the object side to the image side, together.

In the imaging lens of the present disclosure, it is preferable for Conditional Formula (1) below to be satisfied. Note that it is more preferable for Conditional Formula (1-1) below to be satisfied. 0.30<f/fb<0.70  (1) 0.40<f/fb<0.60  (1-1)

wherein f is the focal length of the entire lens system, and fb is the focal length of the rear group.

In addition, it is preferable for Conditional Formula (2) below to be satisfied. Note that it is more preferable for Conditional Formula (2-1) below to be satisfied. 30.00<νd(R1)−νd(R2)<80.00  (2) 35.00<νd(R1)−νd(R2)<75.00  (2-1)

wherein νd(R1) is the Abbe's number of the positive lens of the cemented lens within the rear group, and νd(R2) is the Abbe's number of the negative lens of the cemented lens within the rear group.

In addition, it is preferable for Conditional Formula (3) below to be satisfied. Note that it is more preferable for Conditional Formula (3-1) below to be satisfied. −3.00<f/fn<−0.90  (3) −2.50<f/fn<−1.10  (3-1)

wherein f is the focal length of the entire lens system, and fn is the combined focal length of the at least two negative lenses within the front group (the negative lenses excluding the cemented lens within the front group).

In addition, it is preferable for the front group to have a negative refractive power.

In addition, it is preferable for Conditional Formula (4) below to be satisfied. Note that it is more preferable for Conditional Formula (4-1) below to be satisfied. −0.90<f/fa<0.00  (4) −0.80<f/fa<−0.10  (4-1)

wherein f is the focal length of the entire lens system, and fa is the focal length of the front group.

In addition, it is preferable for Conditional Formula (5) below to be satisfied. Note that it is more preferable for Conditional Formula (5-1) below to be satisfied. 0.00<νd(F1)−νd(F2)<30.00  (5) 0.00<νd(F1)−νd(F2)<23.40  (5-1)

wherein νd(F1) is the Abbe's number of the negative lens of the cemented lens within the front group, and νd(F2) is the Abbe's number of the positive lens of the cemented lens within the front group.

In addition, it is preferable for the rear group to consist of, in order from the object side to the image side, a positive lens, and the cemented lens, formed by cementing a positive lens and a negative lens, provided in this order form the object side to the image side, together.

In addition, the front group may consist of, in order from the object side to the image side, two negative lenses, and the cemented lens formed by cementing a negative lens and a positive lens, provided in this order from the object side to the image side, together. Alternatively, the front group may consist of, in order from the object side to the image side, three negative lenses, and the cemented lens formed by cementing a negative lens and a positive lens, provided in this order from the object side to the image side, together.

An imaging apparatus of the present disclosure is characterized by being equipped with the imaging lens of the present disclosure described above.

Note that the above expression “consists of” means that lenses that practically have no power, optical elements other than lenses such as a stop, a cover glass, and filters, and mechanical components such as lens flanges, a lens barrel, an imaging element, a camera shake correcting mechanism, etc. may be included, in addition to the constituent elements listed above.

Note that the surface shapes, the radii of curvature, and the signs of the refractive powers of lenses in the above lens are those which are considered in the paraxial region for lenses that include aspherical surfaces.

The imaging lens of the present disclosure consists of, in order from the object side to the image side: the front group; the aperture stop; and the rear group having a positive refractive power. The front group consists of, in order from the object side to the image side: at least two negative lenses, and a cemented lens formed by cementing a negative lens and a positive lens having a smaller Abbe's number with respect to the d line (wavelength: 587.6 nm) than the negative lens, provided in this order from the object side to the image side, together. The rear group consists of, in order from the object side to the image side: at least one positive lens, and a cemented lens having a positive refractive power as a whole, formed by cementing a positive lens and a negative lens, provided in this order form the object side to the image side, together. Therefore, it is possible for the imaging lens to favorably correct various aberrations.

In addition, the imaging apparatus of the present disclosure is equipped with the imaging lens of the present disclosure. Therefore, the imaging apparatus of the present disclosure is capable of obtaining images having high image quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional diagram that illustrates the lens configuration of an imaging lens according to an embodiment of the present disclosure (common with an imaging lens of Example 1).

FIG. 2 is a cross sectional diagram that illustrates the lens configuration of an imaging lens according to Example 2 of the present disclosure.

FIG. 3 is a cross sectional diagram that illustrates the lens configuration of an imaging lens according to Example 3 of the present disclosure.

FIG. 4 is a cross sectional diagram that illustrates the lens configuration of an imaging lens according to Example 4 of the present disclosure.

FIG. 5 is a cross sectional diagram that illustrates the lens configuration of an imaging lens according to Example 5 of the present disclosure.

FIG. 6 is a collection of diagrams that illustrate various aberrations of the imaging lens of Example 1.

FIG. 7 is a collection of diagrams that illustrate various aberrations of the imaging lens of Example 2.

FIG. 8 is a collection of diagrams that illustrate various aberrations of the imaging lens of Example 3.

FIG. 9 is a collection of diagrams that illustrate various aberrations of the imaging lens of Example 4.

FIG. 10 is a collection of diagrams that illustrate various aberrations of the imaging lens of Example 5.

FIG. 11 is a diagram that schematically illustrates an imaging apparatus according to an embodiment of the present disclosure.

FIG. 12 is a diagram that schematically illustrates an imaging apparatus according to another embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the attached drawings. FIG. 1 is a cross sectional diagram that illustrates the lens configuration of an imaging lens according to an embodiment of the present disclosure. The example of the configuration illustrated in FIG. 1 corresponds to the configuration of an imaging lens of Example 1 to be described later. In FIG. 1, the left side is the object side, and the right side is the image side. Note that the aperture stop St illustrated in FIG. 1 does not necessarily represent the size or shape thereof, but merely indicates the position of the aperture stop St along an optical axis Z. In addition, FIG. 1 also illustrates an axial light beam wa and a light beam wb at a maximum angle of view.

As illustrated in FIG. 1, this imaging lens is constituted by, in order from the object side to the image side, a front group G1, the aperture stop St, and a rear group G2 having a positive refractive power.

FIG. 1 illustrates an example in which an optical member having a light input surface and a light output surface which are parallel to each other is provided between the rear group G2 and an image formation plane Sim. The optical member PP presumes the presence of an optical path converting prism for bending optical paths, filters, a cover glass, etc. A configuration from which the optical member PP is omitted is also possible in the objective lens for endoscopes of the present disclosure. Note that in the case that an optical path converting prism is employed, optical paths will become bent. However, FIG. 1 is a diagram in which optical paths are expanded, in order to facilitate understanding.

The front group G1 is constituted by, in order from the object side to the image side: at least two negative lenses, and a cemented lens formed by cementing a negative lens and a positive lens having a smaller Abbe's number with respect to the d line (wavelength: 587.6 nm) than the negative lens, provided in this order from the object side to the image side, together.

Providing at least two negative lenses in order from the most object side of the front group G1 in this manner contributes to a widening of the angle of view, and also enables field curvature to be suppressed. In addition, by the cemented lens being the only lens positioned at the image side of the at least two negative lenses within the front group G1, the total length and the diameter of the front group G1 can be prevented from increasing, while sensitivity to eccentricities can be suppressed, and longitudinal chromatic aberration and lateral chromatic aberration can be favorably corrected. Particularly in the case that the imaging lens of the present disclosure is applied to an endoscope, it is preferable for the imaging lens to be compact with a small diameter. Therefore, this aspect of the present disclosure is well suited for such an application.

The rear group is constituted by, in order from the object side to the image side: at least one positive lens, and a cemented lens having a positive refractive power as a whole, formed by cementing a positive lens and a negative lens, provided in this order form the object side to the image side, together.

Providing at least one positive lens in order from the most object side of the rear group G2 in this manner enables spherical aberration to be suppressed. In addition, by providing the cemented lens described above at the most image side of the rear group G2, the cemented lens will become remote from the aperture stop St and the heights at which peripheral light rays pass through the cemented lens will become high. As a result, the effect of correcting lateral chromatic aberration can be increased without excessively correcting longitudinal chromatic aberration. Further, by the cemented lens within the rear group G2 having a positive refractive power, the incident angles of principal rays of light at peripheral angles of view into the image formation plane can be suppressed.

In the imaging lens of the present disclosure, it is preferable for Conditional Formula (1) below to be satisfied. By configuring the imaging lens such that the value of f/fb is not less than or equal to the lower limit defined in Conditional Formula (1), securing back focus and a shortening of the total length of the lens system can both be realized. In addition, by configuring the imaging lens such that the value of f/fb is not greater than or equal to the upper limit defined in Conditional Formula (1), field curvature can be suppressed. Note that more favorable properties can be obtained if Conditional Formula (1-1) below is satisfied. 0.30<f/fb<0.70  (1) 0.40<f/fb<0.60  (1-1)

wherein f is the focal length of the entire lens system, and fb is the focal length of the rear group.

In addition, it is preferable for Conditional Formula (2) below to be satisfied. By configuring the imaging lens such that the value of νd(R1)−νd(R2) is not less than or equal to the lower limit defined in Conditional Formula (2), lateral chromatic aberration can be suppressed. In addition, by configuring the imaging lens such that the value of νd(R1)−νd(R2) is not greater than or equal to the upper limit defined in Conditional Formula (2), longitudinal chromatic aberration can be suppressed. Note that more favorable properties can be obtained if Conditional Formula (2-1) below is satisfied. 30.00<νd(R1)−νd(R2)<80.00  (2) 35.00<νd(R1)−νd(R2)<75.00  (2-1) wherein νd(R1) is the Abbe's number of the positive lens of the cemented lens within the rear group, and νd(R2) is the Abbe's number of the negative lens of the cemented lens within the rear group.

In addition, it is preferable for Conditional Formula (3) below to be satisfied. By configuring the imaging lens such that the value of f/fn is not less than or equal to the lower limit defined in Conditional Formula (3), field curvature can be suppressed. In addition, by configuring the imaging lens such that the value of f/fn is not greater than or equal to the upper limit defined in Conditional Formula (3), the field angle can be increased. Note that more favorable properties can be obtained if Conditional Formula (3-1) below is satisfied. −3.00<f/fn<−0.90  (3) −2.50<f/fn<−1.10  (3-1)

wherein f is the focal length of the entire lens system, and fn is the combined focal length of the at least two negative lenses within the front group (the negative lenses excluding the cemented lens within the front group).

In addition, it is preferable for the front group G1 to have a negative refractive power. By adopting this configuration, back focus can be secured.

In addition, it is preferable for Conditional Formula (4) below to be satisfied. By configuring the imaging lens such that the value of f/fa is not less than or equal to the lower limit defined in Conditional Formula (4), field curvature can be suppressed. In addition, by configuring the imaging lens such that the value of f/fa is not greater than or equal to the upper limit defined in Conditional Formula (4), the field angle can be increased. Note that more favorable properties can be obtained if Conditional Formula (4-1) below is satisfied. −0.90<f/fa<0.00  (4) −0.80<f/fa<−0.10  (4-1)

wherein f is the focal length of the entire lens system, and fa is the focal length of the front group.

In addition, it is preferable for Conditional Formula (5) below to be satisfied. By configuring the imaging lens such that the value of νd(F1)−νd(F2) is not less than or equal to the lower limit defined in Conditional Formula (5), lateral chromatic aberration can be suppressed. In addition, by configuring the imaging lens such that the value of νd(F1)−νd(F2) is not greater than or equal to the upper limit defined in Conditional Formula (5), longitudinal chromatic aberration can be suppressed. Note that more favorable properties can be obtained if Conditional Formula (5-1) below is satisfied. 0.00<νd(F1)−νd(F2)<30.00  (5) 0.00<νd(F1)−νd(F2)<23.40  (5-1)

wherein νd(F1) is the Abbe's number of the negative lens of the cemented lens within the front group, and νd(F2) is the Abbe's number of the positive lens of the cemented lens within the front group.

In addition, it is preferable for the rear group G2 to consist of, in order from the object side to the image side, a positive lens, and the cemented lens, formed by cementing a positive lens and a negative lens, provided in this order form the object side to the image side, together. By configuring the rear group G2 to have three lenses, an imaging lens having a desired level of performance can be constituted by a small number of lenses. Note that the positive lens provided most toward the object side within the rear group G2 exhibits the effect of suppressing spherical aberration. In addition, the cemented lens exhibits the effect of suppressing lateral chromatic aberration.

In addition, the front group G1 may consist of, in order from the object side to the image side, two negative lenses, and the cemented lens formed by cementing a negative lens and a positive lens, provided in this order from the object side to the image side, together. By configuring the front group G1 to have four lenses in this manner, an imaging lens having a desired level of performance can be constituted by a small number of lenses. Note that the two negative lenses provided most toward the object side within the front group G1 exhibits the effect of suppressing field curvature. In addition, the cemented lens exhibits the effect of suppressing longitudinal chromatic aberration and lateral chromatic aberration.

Note that alternatively, the front group G1 may consist of, in order from the object side to the image side, three negative lenses, and the cemented lens formed by cementing a negative lens and a positive lens, provided in this order from the object side to the image side, together. In this case, the number of negative lenses will increase compared to the case described above in which the front group G1 is constituted by four lenses. However, this configuration is advantageous from the viewpoint of widening the angle of view.

In addition, it is preferable for a protective multiple layer film coating to be administered in the case that the present imaging lens is to be utilized in extreme environments. Further, an antireflection coating may be administered in addition to the protective coating, in order to reduce ghost light and the like during utilization of the imaging lens.

In addition, in the case that this imaging lens is applied to an imaging apparatus, it is preferable for a cover glass, prisms, and various filters, such as an infrared cutoff filter and a low pass filter, to be provided between the lens system and an image formation plane Sim, depending on the configuration of the imaging apparatus. Note that these filters may be provided among the lenses instead of being provided between the lens system and the image formation plane Sim. As a further alternative, coatings that exhibit the same effects as these filters may be administered on the lens surfaces of the lenses.

Next, examples of numerical values of the imaging lens of the present disclosure will be described.

First, an imaging lens of Example 1 will be described. FIG. 1 is a cross sectional diagram that illustrates the lens configuration of the imaging lens of Example 1. Note that in FIG. 1 and FIGS. 2 through 5 that correspond to Examples 2 through 5 to be described later, the left side is the object side and the right side is the image side. In addition, the aperture stops St illustrated in FIGS. 1 through 5 do not necessarily represent the sizes and shapes thereof, but only the positions thereof along the optical axis Z.

In the imaging lens of Example 1, the front group G1 is constituted by, in order from the object side to the image side, two negative lenses L1 a and L1 b, and a cemented lens formed by cementing a negative lens L1 c and a positive lens L1 d, provided in this order from the object side to the image side, together.

In addition, the rear group G2 is constituted by, in order from the object side to the image side, a positive lens L2 a and a cemented lens formed by cementing a positive lens L2 b and a negative lens L2 c, provided in this order from the object side to the image side, together.

Basic lens data are shown in Table 1, and data related to various items are shown in Table 2 for the imaging lens of Example 1. The meanings of the symbols in the tables will be described for Example 1 as an example, but the meanings are basically the same for Examples 2 through 5 as well.

In Table 1, surface numbers that sequentially increase with the surface of the constituent element most toward the object side being designated as 1 are listed in the column Surface Number; the radii of curvature of each surface are listed in the column Radius of Curvature; and distances along the optical axis Z between each surface and a surface adjacent thereto are listed in the column Distance. In addition, the refractive indices with respect to the d line (wavelength: 587.6 nm) of each constituent element are listed in the column n; and the Abbe's numbers with respect to the d line of each constituent element are listed in the column v.

Here, the signs of the radii of curvature are positive in cases that the surface shape is convex toward the object side, and negative in cases that the surface shape is convex toward the image side. Table 1 also shows data regarding the aperture stop St and the optical member PP. Text reading “(Stop)” is indicated along with a surface number in the column of the surface number at the surface corresponding to the aperture stop.

The values of the focal length f′ of the entire lens system, the back focus Bf′, the F value F No., and the full angle of view 2ω are shown as data related to various items in Table 2.

In the basic lens data, and the data related to various items, degrees are used as the units for angles and mm are used as the units for lengths. However, it is possible for optical systems to be proportionately enlarged or proportionately reduced and utilized. Therefore, other appropriate units may be used.

TABLE 1 Example 1: Lens Data (n and ν are for the d line) Surface Radius of Number Curvature Distance n ν  1 4.5453 0.2000 1.65160 58.55  2 1.5152 1.1000  3 9.9993 0.2000 1.95375 32.32  4 0.9429 0.5917  5 1.8670 0.3667 1.63854 55.45  6 0.7905 0.5512 1.67270 32.10  7 −7.5719 0.7051  8 (Stop) ∞ 0.3105  9 8.4126 0.2000 1.43875 94.66 10 −1.6153 0.3000 11 1.6666 0.4500 1.43875 94.66 12 −1.1985 0.2000 1.84666 23.78 13 −3.4481 0.1117 14 ∞ 1.5000 1.55920 53.92 15 ∞ 1.5000 1.55920 53.92 16 ∞ 0.3000 1.51633 64.05 17 ∞

TABLE 2 Example 1: Items (d line) f′ 0.94 Bf′ 2.22 F No. 7.97 2ω (°) 219.6

FIG. 6 is a collection of diagrams that illustrate aberrations of the imaging lens of Example 1. In FIG. 6, diagrams that illustrate spherical aberration, astigmatism, and lateral chromatic aberration of the imaging lens of Example 1 in a state focused on an object having a concave surface with a radius of curvature of 0.020 m toward the lens, positioned at a distance of 0.012 m from the lens, are illustrated in this order from the left side of the drawing sheet. The diagrams that illustrate spherical aberration and astigmatism show aberrations related to the d line (wavelength: 587.6 nm). The diagram that illustrates spherical aberration shows aberrations related to the d line (wavelength: 587.6 nm), the C line (wavelength: 656.3 nm), and the F line (wavelength: 486.1 nm), as a black solid line, a long broken line, and a dotted line, respectively. In the diagram that illustrates astigmatism, aberrations in the sagittal direction and aberrations in the tangential direction related to the d line are indicated by a solid line and a dotted line, respectively. In the diagram that illustrates lateral chromatic diagram, aberrations related to the C line (wavelength: 656.3 nm) and the F line (wavelength: 486.1 nm) are shown as a long broken line and a dotted line, respectively. In the diagram that illustrates spherical aberration, “F No.” denotes the F number. In the other diagrams that illustrate the aberrations, ω denotes half angles of view.

The symbols, the meanings, and the manners in which the various pieces of data are described in the description of Example 1 above are the same for the examples to be described below unless otherwise noted. Therefore, redundant descriptions will be omitted hereinafter.

Next, an imaging lens of Example 2 will be described. FIG. 2 is a cross sectional diagram that illustrates the lens configuration of the imaging lens of Example 2. The imaging lens of Example 2 has the same lens configuration as the imaging lens of Example 1. Basic lens data are shown in Table 3, and data related to various items are shown in Table 4 for the imaging lens of Example 2. In addition, FIG. 7 is a collection of diagrams that illustrate various aberrations of the imaging lens of Example 2.

TABLE 3 Example 2: Lens Data (n and ν are for the d line) Surface Radius of Number Curvature Distance n ν  1 2.7777 0.2000 1.65160 58.55  2 1.3333 1.2000  3 10.4104 0.2000 1.95375 32.32  4 0.9723 0.8677  5 2.9377 0.2000 1.63854 55.45  6 0.8913 0.5000 1.67270 32.10  7 −4.7767 0.7740  8 (Stop) ∞ 0.2503  9 −165.5932 0.2000 1.43875 94.66 10 −1.4144 0.2081 11 1.5807 0.4500 1.43875 94.66 12 −1.0839 0.2000 1.84666 23.78 13 −3.2346 0.0500 14 ∞ 1.5000 1.55920 53.92 15 ∞ 1.5000 1.55920 53.92 16 ∞ 0.3000 1.51633 64.05 17 ∞

TABLE 4 Example 2: Items (d line) f′ 0.85 Bf′ 2.17 F No. 6.66 2ω (°) 220.0

Next, an imaging lens of Example 3 will be described. FIG. 3 is a cross sectional diagram that illustrates the lens configuration of the imaging lens of Example 3. The imaging lens of Example 3 has the same lens configuration as the imaging lens of Example 1. Basic lens data are shown in Table 5, and data related to various items are shown in Table 6 for the imaging lens of Example 3. In addition, FIG. 8 is a collection of diagrams that illustrate various aberrations of the imaging lens of Example 3.

TABLE 5 Example 3: Lens Data (n and ν are for the d line) Surface Radius of Number Curvature Distance n ν  1 5.6177 0.3006 1.43875 94.66  2 0.7505 0.4438  3 −2.8257 0.2000 2.00100 29.13  4 0.9667 0.2745  5 1.0667 0.8659 1.95375 32.32  6 0.5000 0.6350 1.72151 29.23  7 −2.3753 0.0500  8 (Stop) ∞ 0.0855  9 1.1679 0.8487 1.43875 94.66 10 −1.0925 0.2000 11 −4.8806 0.4000 1.48749 70.24 12 −0.6721 0.2000 2.00100 29.13 13 −1.5904 0.2000 14 ∞ 1.5000 1.55920 53.92 15 ∞ 1.5000 1.55920 53.92 16 ∞ 0.3000 1.51633 64.05 17 ∞

TABLE 6 Example 3: Items (d line) f′ 1.09 Bf′ 2.28 F No. 9.19 2ω (°) 199.0

Next, an imaging lens of Example 4 will be described. FIG. 4 is a cross sectional diagram that illustrates the lens configuration of the imaging lens of Example 4. The imaging lens of Example 4 has the same lens configuration as the imaging lens of Example 1. Basic lens data are shown in Table 7, and data related to various items are shown in Table 8 for the imaging lens of Example 4. In addition, FIG. 9 is a collection of diagrams that illustrate various aberrations of the imaging lens of Example 4.

TABLE 7 Example 4: Lens Data (n and ν are for the d line) Surface Radius of Number Curvature Distance n ν  1 7.3449 0.1967 1.59551 39.24  2 0.8614 0.4623  3 −2.4876 0.2404 1.85135 40.10  4 1.0726 0.2500  5 1.1062 0.6968 1.95375 32.32  6 0.5089 0.6224 1.69895 30.13  7 −2.3459 0.1092  8 (Stop) ∞ 0.1055  9 1.1951 0.8180 1.43875 94.66 10 −1.0920 0.2040 11 −4.5653 0.5079 1.51633 64.14 12 −0.6673 0.2948 2.00100 29.13 13 −1.6536 0.2493 14 ∞ 1.5000 1.55920 53.92 15 ∞ 1.5000 1.55920 53.92 16 ∞ 0.3000 1.51633 64.05 17 ∞

TABLE 8 Example 4: Items (d line) f′ 1.12 Bf′ 2.32 F No. 9.35 2ω (°) 192.6

Next, an imaging lens of Example 5 will be described. FIG. 5 is a cross sectional diagram that illustrates the lens configuration of the imaging lens of Example 5. The imaging lens of Example 5 differs from the imaging lens of Example 1 in the point that a front group G1 is constituted by, in order from the object side to the image side, three negative lenses L1 a, L1 b, and L1 c, and a cemented lens formed by cementing a negative lens L1 d and a positive lens L1 e, provided in this order from the object side to the image side, together. Basic lens data are shown in Table 9, and data related to various items are shown in Table 10 for the imaging lens of Example 5. In addition, FIG. 10 is a collection of diagrams that illustrate various aberrations of the imaging lens of Example 5.

TABLE 9 Example 5: Lens Data (n and ν are for the d line) Surface Radius of Number Curvature Distance n ν  1 8.3991 0.2000 1.83481 42.72  2 2.1520 3.0681  3 −2.6133 0.9609 1.65100 56.16  4 −9.1407 1.1319  5 1.8061 0.4000 1.75700 47.82  6 1.4247 0.5649  7 3.6108 0.2000 1.63854 55.45  8 1.8960 0.3000 1.67270 32.10  9 −19.9698 0.1119 10 (Stop) ∞ 0.2068 11 2.7095 1.4550 1.43875 94.66 12 −2.0565 0.0500 13 3.0055 0.4846 1.43875 94.66 14 −1.6024 0.6172 1.84666 23.78 15 −32.2551 0.0500 16 ∞ 1.5000 1.55920 53.92 17 ∞ 1.5000 1.55920 53.92 18 ∞ 0.3000 1.51633 64.05 19 ∞

TABLE 10 Example 5: Items (d line) f′ 1.05 Bf′ 2.14 F No. 3.28 2ω (°) 226.0

Table 11 shows values corresponding to Conditional Formulae (1) through (5) for the imaging lenses of Examples 1 through 5. Note that all of the Examples use the d line as a reference wavelength, and the values shown in Table 11 below are those for the reference wavelength.

TABLE 11 Example Example Example Example Example Formula Condition 1 2 3 4 5 (1) f/fb 0.451 0.403 0.596 0.597 0.499 (2) vd(R1)-vd(R2) 70.875 70.875 41.101 35.007 70.875 (3) f/fn −1.385 −1.171 −2.481 −2.417 −1.366 (4) f/fa −0.378 −0.296 −0.793 −0.829 −0.137 (5) vd(F1)-vd(F2) 23.350 23.350 3.086 2.191 23.350

As can be understood from the above data, all of the imaging lenses of Examples 1 through 5 satisfy Conditional Formulae (1) through (5), and are imaging lenses in which various aberrations are favorably corrected.

Next, imaging apparatuses according to embodiments of the present disclosure will be described. First, an example of a case in which an endoscope is the embodiment of an imaging apparatus of the present disclosure will be described. FIG. 11 is a diagram that illustrates the schematic structure of the entirety of the endoscope.

The endoscope 100 illustrated in FIG. 11 is mainly equipped with an operating portion 102, an insertion portion 104, and a universal cord 106 to be connected to a connector portion (not shown). The majority of the insertion portion 104 is a flexible portion 107 which can be bent in desired directions along an insertion path. A bendable portion 108 is linked to the leading end of the flexible portion 107, and a leading end portion 110 is linked to the leading end of the bendable portion 108. The bendable portion 108 is provided to orient the leading end portion 110 in desired directions. Bending operations are enabled by rotating a bending operation knob 109 provided on the operating portion 102. An imaging lens 1 according to an embodiment of the present disclosure is provided within the interior of the leading end portion 110. Note that the imaging lens 1 is schematically illustrated in FIG. 11. The endoscope of the present embodiment is equipped with the imaging lens 1 of the present disclosure. Therefore, the endoscope is capable of obtaining favorable images.

Next, an example of a case in which a vehicle mounted camera is the embodiment of an imaging apparatus of the present disclosure will be described. FIG. 12 is a diagram that illustrates the manner in which vehicle mounted cameras are mounted on an automobile.

In FIG. 12, an automobile 200 is equipped with an externally mounted camera 201 for imaging a blind spot range at the side surface on the side of the passenger seat, an externally mounted camera 202 for imaging a blind spot range at the rear side of the automobile 200, and a internally mounted camera 203 which is mounted on the back surface of the rear view mirror and images the same range as the field of view of a driver. The externally mounted camera 201, the externally mounted camera 202, and the internally mounted camera 203 are imaging apparatuses according to an embodiment of the present disclosure, and are equipped with imaging lenses according to an embodiment of the present disclosure and imaging elements that convert optical images formed by the imaging lenses into electrical signals. The vehicle mounted cameras of the present embodiment (the externally mounted cameras 201 and 202, as well as the internally mounted camera 203) are equipped with the imaging lens of the present disclosure. Therefore, the vehicle mounted cameras are capable of obtaining favorable images.

The present disclosure has been described with reference to the embodiments and Examples. However, the present disclosure is not limited to the above embodiments and Examples, and various modifications are possible. For example, the numerical values of the radii of curvature, the surface distances, the refractive indices, the Abbe's numbers, etc. of the lens components are not limited to those exemplified in the above Examples, and may be different values. 

What is claimed is:
 1. A fixed focal length imaging lens consisting of, in order from the object side to the image side: a front group; an aperture stop; and a rear group having a positive refractive power; the front group consisting of, in order from the object side to the image side: at least two negative lenses, and a cemented lens formed by cementing a negative lens and a positive lens having a smaller Abbe's number with respect to the d line than the negative lens, provided in this order from the object side to the image side, together; and the rear group consisting of, in order from the object side to the image side: at least one positive lens, and a cemented lens having a positive refractive power as a whole, formed by cementing a positive lens and a negative lens, provided in this order form the object side to the image side, together, wherein Conditional Formula (1) below is satisfied: 0.30<f/fb<0.60  (1), wherein f is the focal length of the entire lens system, and fb is the focal length of the rear group, and Conditional Formula (2) below is satisfied: 30.00<νd(R1)−νd(R2)<80.00  (2), wherein νd(R1) is the Abbe's number of the positive lens of the cemented lens within the rear group, and νd(R2) is the Abbe's number of the negative lens of the cemented lens within the rear group.
 2. An imaging lens as defined in claim 1, in which Conditional Formula (3) below is satisfied: −3.00<f/fn<−0.90  (3) wherein f is the focal length of the entire lens system, and fn is the combined focal length of the at least two negative lenses within the front group.
 3. An imaging lens as defined in claim 1, wherein: the front group has a negative refractive power.
 4. An imaging lens as defined in claim 1, in which Conditional Formula (4) below is satisfied: −0.90<f/fa<0.00  (4) wherein f is the focal length of the entire lens system, and fa is the focal length of the front group.
 5. An imaging lens as defined in claim 1, in which Conditional Formula (5) below is satisfied: 0.00<νd(F1)−νd(F2)<30.00  (5) wherein νd(F1) is the Abbe's number of the negative lens of the cemented lens within the front group, and νd(F2) is the Abbe's number of the positive lens of the cemented lens within the front group.
 6. An imaging lens as defined in claim 1, wherein: the rear group consists of, in order from the object side to the image side, a positive lens, and the cemented lens, formed by cementing a positive lens and a negative lens, provided in this order form the object side to the image side, together.
 7. An imaging lens as defined in claim 1, wherein: the front group consists of, in order from the object side to the image side, two negative lenses, and the cemented lens formed by cementing a negative lens and a positive lens, provided in this order from the object side to the image side, together.
 8. An imaging lens as defined in claim 1, wherein: the front group consists of, in order from the object side to the image side, three negative lenses, and the cemented lens formed by cementing a negative lens and a positive lens, provided in this order from the object side to the image side, together.
 9. An imaging lens as defined in claim 1, in which Conditional Formula (2-1) below is satisfied: 35.00<νd(R1)−νd(R2)<75.00  (2-1) wherein νd(R1) is the Abbe's number of the positive lens of the cemented lens within the rear group, and νd(R2) is the Abbe's number of the negative lens of the cemented lens within the rear group.
 10. An imaging lens as defined in claim 1, in which Conditional Formula (3-1) below is satisfied: −2.50<f/fn<−1.10  (3-1) wherein f is the focal length of the entire lens system, and fn is the combined focal length of the at least two negative lenses within the front group.
 11. An imaging lens as defined in claim 1, in which Conditional Formula (4-1) below is satisfied: −0.80<f/fa<−0.10  (4-1) wherein f is the focal length of the entire lens system, and fa is the focal length of the front group.
 12. An imaging lens as defined in claim 1, in which Conditional Formula (5-1) below is satisfied: 0.00<νd(F1)−νd(F2)<23.40  (5-1) wherein νd(F1) is the Abbe's number of the negative lens of the cemented lens within the front group, and νd(F2) is the Abbe's number of the positive lens of the cemented lens within the front group.
 13. An imaging apparatus equipped with a fixed focal length imaging lens, the imaging lens consisting of, in order from the object side to the image side: a front group; an aperture stop; and a rear group having a positive refractive power; the front group consisting of, in order from the object side to the image side: at least two negative lenses, and a cemented lens formed by cementing a negative lens and a positive lens having a smaller Abbe's number with respect to the d line than the negative lens, provided in this order from the object side to the image side, together; and the rear group consisting of, in order from the object side to the image side: at least one positive lens, and a cemented lens having a positive refractive power as a whole, formed by cementing a positive lens and a negative lens, provided in this order form the object side to the image side, together, wherein Conditional Formula (1) below is satisfied: 0.30<f/fb<0.60  (1), wherein f is the focal length of the entire lens system, and fb is the focal length of the rear group, and Conditional Formula (2) below is satisfied: 30.00<νd(R1)−νd(R2)<80.00  (2), wherein νd(R1) is the Abbe's number of the positive lens of the cemented lens within the rear group, and νd(R2) is the Abbe's number of the negative lens of the cemented lens within the rear group. 